Polyesters, in particular, polyethylene terephthalate, have excellent mechanical properties, such as crystallinity and strength, chemical resistance, thermal properties, electrical properties, and transparency, and are thus used in various industrial applications, such as films, fibers, bottles, and extrusion-molded products. The demand therefor is growing. In particular, polyesters are used in the field of film applications, for example, magnetic recording applications, agricultural applications, packaging applications, capacitor applications, and construction materials applications where there are massive demands, due to their superior mechanical properties and economic efficiency.
For example, polyethylene terephthalate is produced from ethylene glycol and terephthalic acid or its ester-forming derivative. In general, commercial processes for producing polymers with high molecular weight commonly use antimony-based compounds as polycondensation catalysts.
However, antimony-based compounds have the following problems.
For example, antimony-based compounds partly evaporate and dissipate during polymer melting processes and their residues deposit around the dies and induce surface defects in films.
Antimony-based compounds tend to form relatively large particles in polymers and thus cause an increased filtration pressure at filters, surface defects in films during film-making, and breaking of films under severe conditions.
Use of antimony-based compounds impairs the stability of particles added to polymers, thereby causing agglomeration of particles. This results in decreased quality, such as generation of coarse projections in surfaces, and decreased operation efficiency due to increased filtration pressures at filters, as described above.
Accordingly polyesters containing small amounts of antimony or free of antimony are desired.
Known examples of compounds other than antimony-based compounds that can function as polycondensation catalysts are germanium-based compounds. However, germanium-based compounds are quite expensive and thus not suitable for common usage.
Other than antimony-based compounds and germanium-based compounds, titanium-based compounds are known.
For example, Japanese Unexamined Patent Application Publication No. 2002-187942 discloses a method of processing a titanium compound with water containing an alkaline compound, an organic solvent, or a mixture thereof before adding the titanium compound to the reaction system in the polyester polymerization step.
Japanese Unexamined Patent Application Publication No. 2000-119383 discloses a polyester polymerization catalyst composed of titanium dioxide having an average primary particle diameter of 100 nm or less.
Japanese Unexamined Patent Application Publication No. 2000-17065 discloses a polyester composition composed of a high-purity dicarboxylic acid component and a titanium compound.
Japanese Unexamined Patent Application Publication No. 10-316749 discloses a method for producing a polyester resin in which a product prepared by heating a mixture of an organotitanium compound and an organotin compound is used as a catalyst.
Japanese Unexamined Patent Application Publication No. 63-278927 discloses a method for producing polyester using a particular amount of a manganese compound, and an alkali metal compound, a phosphorus compound, or an organotitanium compound.
Japanese Unexamined Patent Application Publication No. 54-43294 discloses a method for producing a polyester from a zinc compound, a cobalt compound, an aromatic multivalent carboxylic acid, and tetraalkyl titanate.
Japanese Unexamined Patent Application Publication No. 54-37196 discloses a method for producing polyester, including performing transesterification between a particular amount of a manganese compound and a cobalt compound and then performing polymerization in the presence of a catalyst, which is a reaction product between an aromatic multivalent carboxylic acid and tetraalkyl titanate.
Japanese Unexamined Patent Application Publication No. 51-81896 discloses a method of combining a tellurium compound, a cobalt compound, and a cobalt salt of a phosphorus compound in the presence of a titanium oxide acting as a catalyst.
Japanese Unexamined Patent Application Publication No. 51-81895 discloses a method of combining a bismuth compound, a cobalt compound, and a cobalt salt of a phosphorus compound in the presence of a titanium oxide acting as a catalyst.
Japanese Unexamined Patent Application Publication No. 51-66395 discloses a method of adding a nickel compound in the presence of a titanium oxide acting as a catalyst.
Japanese Unexamined Patent Application Publication No. 7-292087 discloses a polyester in which the content of the metal precipitated particles derived from a titanium catalyst is controlled to a particular value or lower.
However, these techniques cannot prevent titanium compounds (polyester polymerization catalyst) from forming debris by deterioration or agglomeration during polyester polymerization reaction. Although it is possible to reduce debris, these techniques cannot prevent formation of coarse particles. For example, these techniques do not achieve sufficient effects as films for magnetic recording media applications that require particularly smooth and flat surfaces.
Films for magnetic recording media or the like commonly contain particles for enhancing the film slidability and film surface properties. For example, Japanese Unexamined Patent Application Publication No. 59-217755 discloses that the use of organic polymer particles highly compatible with polyesters is preferable. However, this technique is not sufficient for reducing coarse projections in film surfaces.
When titanium-based compounds are used as polycondensation catalysts, yellow polyesters are obtained as a result. Moreover, thermal stability of the polymer melted by heating is decreased, and film ruptures and the like occur, thereby decreasing productivity.
In order to overcome the problem of yellow coloration, cobalt compounds are commonly added to polyesters to reduce yellowness; however, cobalt compounds decrease thermal stability of the polyesters. Accordingly, the thermal stability of the melted polymer is further decreased, and the productivity is also decreased.
In order to overcome the problem of heat resistance and color tone associated with titanium-based catalysts, International Publication No. 95/18839 pamphlet discloses a method that uses a complex oxide of titanium and silicon as a catalyst.
Japanese Unexamined Patent Application Publication No. 2001-89557 discloses a titanium-compound catalyst synthesized by hydrolyzing a titanium halide.
However, these techniques do not improve hue as much as to be sufficient for films for optical applications, for example, where even minute differences in hue should be avoided. Moreover, the dispersibility of the hue-adjusting agent is low, thereby leading to problems such as scattering and generation of debris.
The present invention aims to provide a practical polyester that can be prepared substantially without using an antimony-based compound as a polycondensation catalyst.